1. Field of the Invention
The present invention relates generally to gas turbine engine compressor outlet guide vane stages and diffusers and, more specifically, to support of the outlet guide vane stage.
2. Background Art
A conventional gas turbine engine includes in serial flow communication a compressor, a discharge flowpath having a compressor outlet guide vane stage including compressor outlet guide vanes (OGVs) disposed between annular inner and outer walls which in turn are mounted in an OGV support structure mechanically tied into an engine casing. Outlet guide vanes typically have airfoil like cross-sections that include a leading edge, a relatively thick middle section, and a thin trailing edge. Downstream of the OGV stage is a combustor diffuser, a combustor, a turbine nozzle, and a high pressure turbine. Typically OGV stage inner and outer walls are supported by corresponding inner and outer annular diffuser inlet walls to form a relatively leak free flowpath therebetween and support the OGV stage. Such a design is illustrated in U.S. Pat. No. 4,483,149, entitled "Diffuser Case for A Gas Turbine Engine", by Gerald R. Rider et al, which issued on Nov. 20, 1984. Some other constructions welded corresponding inner and outer OGV walls and diffuser casings.
During engine operation, the compressor compresses inlet airflow, which is therefore heated thereby. The discharged compressed and heated airflow is then channeled through the OGVs and the diffuser to the combustor wherein it is conventionally mixed with fuel and ignited to form combustion gases. The combustion gases are channeled through the turbine nozzle to the high pressure turbine which extracts energy therefrom for rotating and powering the compressor.
A typical conventional engine has a support assembly for the OGVs and the combustor diffuser which includes an annular inner support extending downstream to the turbine nozzle which may be used to help support the turbine nozzle. An annular outer support extends radially outwardly from the OGVs and the diffuser and is fixedly connected to the casing surrounding the engine for supporting the OGVs and the diffuser.
The turbine nozzle includes a plurality of circumferentially spaced and angled nozzle vanes which conventionally direct the combustion gases into the high pressure turbine. A pressure drop exists across the turbine nozzle and the inner support which generates an axial force which is carried upstream through the inner support, the discharge flowpath, and the outer support to the casing. Since the nozzle vanes are angled, a circumferential component of force is also generated from the combustion gases which results in a torque relative to the engine centerline axis also being transmitted upstream through the inner support and the outer support to the casing.
During an engine thermal transient such as, for example, throttle push, the compressor OGVs and combustor diffuser experience relatively high and nearly instantaneous temperature change due to the relatively hot compressed airflow being discharged from the compressor. Although the inner support responds relatively quickly with the OGVs and the diffuser, the outer support and casing respond relatively slowly to the temperature change. Therefore, the OGVs and diffuser expand more rapidly relative to the outer support which outer support tends to restrain the radial growth thereof resulting in relatively high thermally induced stress at the interface thereof. To alleviate the problems due to this stress a new structural support was developed and is disclosed in a related U.S. Pat. application No. 07/729,956, entitled "Compressor Discharge Flowpath", filed on Jul. 15, 1991, and assigned to the same assignee.
The outer support is typically an annular, conical or cylindrical, surface of revolution or shell, which is relatively stiff requiring relatively large forces to cause deflection thereof. The relatively large thermal mass of the OGVs and combustor diffuser create both a radially outward deflection and rotation of the end of the relatively slowly expanding outer support connected thereto, with attendant large thermal stresses therein. In other words, the supporting end of the outer support shell is caused by the expanding OGVs and diffuser to both expand and twist radially outwardly relative to the outer support shell at distances away from its interface with the OGVs and the diffuser.
Accordingly, the relatively quickly expanding OGVs and diffuser expand radially outwardly to a greater extent than the relatively slowly expanding outer support shell resulting in a differential thermal movement, or expansion, therebetween. Furthermore, there exists a large thermal growth differential between the OGV's and the diffuser due to the disposition of the OGV's in front of the diffuser. The forward position and higher airflow velocities through the OGVs than through the diffuser results in the OGVs heating up quicker than the diffuser during engine acceleration such as during takeoff. This causes a thermal differential movement or growth between the OGVs and the diffuser. This differential thermal movement is accommodated by the bending of the outer support shell of the compressor diffuser at its welded intersection with the OGV's inner and outer platform trailing edges and the compressor diffuser resulting in high thermal stress therein. This induces high stresses at the trailing edges of the outlet guide vanes leading to cracking and premature failure. One method of alleviating this stress is to sector the OGV assembly but this has the disadvantage of creating flow leakage paths.